1. Field of the Invention
The present invention relates to a loop control apparatus for general process control such as flow rate control, pressure control, temperature control of fluid including water and air, and more particularly to a loop control apparatus provided with a function for designing loop control functions.
2. Description of Prior Art
Conventionally, both instrument design and operational equation design are necessary to enable a loop control apparatus to have a wide variety of functions of loop control. The instrument design represents defining and registering various parameters with respect to meters or instruments as elements for the control, and includes: setting values of filter elements; setting MIN and MAX values of physical or engineering quantity that correspond to the range of analog inputs; assigning output port numbers to the analog inputs/outputs; setting PID parameters for PID control such as PB (proportional band), TI (integrated time), TD (derivative time); specifying names of the meters and setting units of the engineering quantity. The setting MIN and MAX values of the engineering quantity is referred to a procedure in which, for example, for an analog input of voltages of 1-5 volts representing temperatures of 0-100 degrees Celsius, 1 V is assigned 0 degrees and 5 V is assigned 100 degrees. The purpose of assigning the port numbers for the analog inputs/outputs is to assign a specific hardware port number to each analog quantity. The operational equation design represents defining specific operations or tasks of the respective instruments, and includes: reading an analog input and translating it into an engineering quantity by means of an A/D converter; determining target values; calculating a manipulate value MV through PID algorithm on the basis of SV, PV, PID parameters; and converting the manipulate value MV into an analog quantity and outputting to the loop under control.
SV is a target value or set value to which the loop system is to be controlled, PV is a process value i.e., actually measured or obtained value through external sensors etc., MV is a manipulate value by which the physical quantity of the loop system is to be increased or decreased with respect to the set value SV. PB, TI, TD are called proportional band, integrated time, and derivative time, respectively, and are used in operation for PID or I-PD.
Conventionally, two methods of loop control apparatus having the described design functions have been used. A first method is a type in which a single computer performs a man-machine interface for designing the loop control functions as for well as execution of control procedures. Displaying and checking data inputs in the interface mode, an operator can perform the instrument design and the operational equation design. A CRT and a Key board used for monitoring purpose in the man-machine interface mode are idle during the control mode of the loop, thus being uneconomical.
A second method is a type in which the man-machine interface is performed by one computer and the control of the loop is effected by another. The instrument design and operational equation design are effected through the "interface computer" which checks design data and communicates the various design data with the "control computer".
With the stated prior art loop control apparatus, producing trains of characters for designing a loop control function requires an application software for formatting the characters. One major problem which has been encountered in connection with the prior art application software method is that determining of PV, SV, and MV or specifying the input/output operation is performed through "fill-in-the-blank", which requires a graphical picture such as an input field in the form of a box or frame into which the user fills the respective parameters. A plurality of such graphical pictures are required for many systems to be controlled except a simple loop where determination of PV, SV, MV or input/output operation will not vary during the loop control. The software for this purpose is different from model to model unless the models are of the same series of the same supplier.